The invention concerns a device as well as a tomographic method, in particular for single-photon (emission) tomography (SPECT).
Single-photon tomography as well as its devices relates to a method of three-dimensional imaging of radiopharmaceuticals inserted in an object. Such objects are in particular persons, animals, plants or parts thereof, as well as inanimate objects.
The radiopharmaceuticals inserted in the object emit photons that are detected and analyzed by the device. As a result of the analysis the position, that is the spatial distribution of the radiopharmaceuticals in the object, is displayed. The position of the radiopharmaceuticals in turn allows conclusions on the object, as for example on a distribution of tissue in the object.
A common device for carrying out a SPECT comprises a gamma camera as detector and an upstream collimator. In general the collimator is a plate composed of a material with a high absorption coefficient and a large number of channels running perpendicularly through the plate. Providing these channels guarantee that only perpendicularly entering photons are detected so that analyzing the local resolution is possible.
SPECT and Positron Emission Tomography (PET) are instruments for the quantitative and in vivo display and reconstruction of spatial radiotracer distribution. Outside of human medicine these methods can be used to develop and evaluate novel tracer combinations in pharmacological and preclinical research. While in PET, various systems for the examination of small laboratory animals are available, such developments so far were insufficient in the field of SPECT, even though TC-99m and I-123 labeled radiopharmaceuticals are of much higher importance in nuclear medicine than PET nuclides.
In order to improve the local resolution and sensitivity of SPECT a hole collimator is used. A hole collimator is characterized by one single hole in the collimator plate through which photons pass. If the object is located closer to the hole collimator than the surface of a gamma camera or detector, a higher resolution of the object can be achieved. The photons do not pass through the hole collimator in an exclusively perpendicular way, but are displayed by an advantageously enlarging central geometry. Thereby a reconstructed resolution that is advantageously distinctly smaller than the detector's own resolution can be achieved.
The photons pass the hole collimator through a small opening, or hole in order to achieve a good local resolution. The smaller the hole, the less photons pass through that hole. Hence the sensitivity of the device is disadvantageously reduced with an increasingly smaller hole. Sensitivity is defined as relationship between the measured counting rate and the activity in the object. If the sensitivity decreases too much, no SPECT imaging can be performed any more. Increasingly smaller holes result in an advantageously smaller local resolution so that regarding the size of the holes a compromise between sensitivity and local resolution has to be found.
A device with a multi-pinhole collimator and detector to register photons passing through the multi-pinhole collimator is known from the DE 101 42 421 (U.S. Pat. No. 7,199,371). The collimator thus has several openings. Hence the distribution of radiopharmaceuticals can be measured with high resolution and high sensitivity. Different distributions of the radiopharmaceuticals in the object are estimated using an iterative reconstruction method, consequent measurement results are calculated that would cause the estimated distributions and as reconstruction result the estimated distribution whose calculated measurement result mostly coincides with the obtained measurement result is selected.
Camera and collimator rotate around the object during SPECT examinations (R-SPECT). The detectors characteristically surround the object with a given radius in 6 degree intervals so that 60 projections for all detectors for one sequence are obtained. In addition to the projections and the angle indication of the detectors changing relatively to the object, the rotation radius of the detectors rotating around the object is a further relevant parameter for the reconstruction. This rotation radius remains constant throughout the whole measurement procedure.
If small objects, mice for instance, are examined, it is also possible to rotate them around their axes and to keep the one or more detector(s) as well as their collimators fixed.
Such methods are of an observational accuracy of approx. 1 millimeter or 1/10 degree.
Thanks to SPECT-examinations a high amount of projection data is available. The information received regarding the object allows an analysis of the position of the radiopharmaceuticals in the object. Functional conclusions for example regarding the coronary blood flow or the receptor density in the brain can then be drawn from the reconstructed, three-dimensional activity patterns.
A disadvantage is the mechanical system for positioning of the detectors, as the mass of detectors can weigh 100 kg and more. The necessary data of some organs that are hard to access can only be obtained under difficult circumstances and with poor results.
A disadvantageous impact on small animals rotating around their own axis consists in the fact that physiological stress in inflicted upon the animal. Furthermore the displacement of soft tissue inside the animal has to be compensated.